Heat pump motor optimization and sensor fault detection

ABSTRACT

The microprocessor-based HVAC control system detects sensor faults by comparing thermistor readings with predetermined extreme values, indicative of sensor fault. The system automatically selects the appropriate combination of automatic/preset modes of operating key system components such as the expansion valve, the demand defrost system and the indoor fan speed control system. With a view towards minimizing any negative impact on system performance and reliability in the event of sensor malfunction.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates generally to heat pump equipment forheating and cooling applications. More particularly, the inventionrelates to an electronic control system for optimizing compressor motorperformance and for handling sensor faults while maintaining a minimumnegative impact on system performance and reliability.

Conventionally air-conditioning and heat pump systems must be designedto handle extreme conditions which only infrequently occur. There is,for example, the maximum load/low voltage condition which occurs only onvery hot days. On very hot days, the compressor discharge pressure ishigh and the suction pressure on the load side (indoors) is also high.High compressor motor torque is therefore required to meet these extremepressure demands, which in turn dictates that the system be designedwith an oversized compressor motor. Exacerbating this condition is thefrequent low voltage or brownout condition that occurs in many parts ofthe country during hot days. The low voltage or brownout conditionplaces an additional strain on the motor.

As a result of these maximum load/low voltage demands, systemmanufacturers have traditionally designed their systems with largermotors than would otherwise be required, if the extreme conditions couldbe guaranteed never to occur. The need to use these larger motorsincreases the cost of the system. Moreover, the use of larger motorsactually decreases the overall system efficiency, since a motor designedto handle the high torque, low voltage extremes typically does notoperate at peak efficiency under the less extreme conditions normallyencountered. In effect, the need to accommodate the maximum load and lowvoltage conditions constrains the system designer to use a lessefficient motor.

With the continuing effort to improve system efficiency, under extremeconditions and normal conditions alike, there is a need for ways tooptimize motor efficiency. Conventional heating and cooling systems havebeen generally deficient in this regard.

Another area where heating and cooling systems could be improved is inthe detection and handling of sensor faults. Today's HVAC equipment isbecoming comparatively complex. Most systems use one or more sensors ortransducers in conjunction with a control system which is intended tokeep the system operating efficiently while meeting the heating andcooling demands of the load. Sensor malfunctions are therefore asignificant problem.

In a conventional heat pump system, for example, a sensor may be used toimplement the defrost function of the outdoor coil in heating mode. Ifthis sensor malfunctions, the heat pump system can begin to improperlyperform the defrost function. This can lead to blockage of the outdoorcoil from frost buildup resulting in significant loss in heatingperformance. Such a conventional system does not have the capability totake corrective action or to alert the user that a sensor malfunctionhas occurred. Thus, a sensor malfunction may not become apparent untilthere is visual inspection of the outdoor unit of the heat pump system,or until the user notices higher electric bills from the lost heatingperformance, which must be made up by electric resistance heaters.

The present invention addresses this and other sensor fault problems byproviding a microprocessor-controlled system operating mechanism whichmonitors the integrity of system sensors and keeps the heat pump systemoperational in alternate modes each designed to have a minimum negativeimpact on system performance and reliability. The system operatingmechanism automatically selects the mode of operation, based on whichsensor or sensors have been found to be malfunctioning. In addition, thesystem operating mechanism also provides an early warning to the user bydisplaying a sensor malfunction code or codes on the room thermostat. Inaddition to alerting the home owner that an error has occurred, themalfunction code is a time saving diagnostic tool for the technicianduring servicing of the system.

Accordingly, in one aspect the invention provides a system operatingmethod in which the discharge temperature of the refrigerant dischargedfrom the compressor is obtained and compared with a predeterminedtemperature indicative of an alert condition. Based on the comparingstep, if the discharge temperature is above the predeterminedtemperature, the discharge temperature is used to control the setting ofthe heat pump system expansion valve. On the other hand, if thedischarge temperature is not above the predetermined temperature (O°F.), the electronic expansion valve (EXV) is set to a predeterminedsetting.

According to another aspect of the invention, a sensor is used to obtainan outdoor air temperature and this outdoor air temperature is comparedwith a predetermined temperature indicative of a sensor fault condition.Based on the comparing step, if the outdoor air temperature is above thepredetermined temperature, the outdoor air temperature is used tocontrol the speed of the indoor fan or blower. On the other hand, if theoutdoor air temperature is not above the predetermined temperature (-77°F.), the fan is run at a predetermined speed and the EXV set to apredetermined opening.

In yet another aspect of the invention, a sensor is used to obtain anoutdoor coil temperature and this temperature is compared with apredetermined temperature indicative of an alert condition. Based on thecomparing step, if the outdoor coil temperature is above thepredetermined temperature, the outdoor coil temperature is used tocontrol operation of the coil defrosting system. On the other hand, ifthe outdoor coil temperature is not above the predetermined temperature(-77° F.), the defrosting system is periodically operated atpredetermined time intervals.

The aforementioned sensor fault handling methods may be implementedseparately or in various combinations depending upon the complexity ofthe heat pump system. As more fully set forth below, the sensor faulthandling methods can be applied in both heating mode and cooling mode.

In yet another aspect of the invention, a system operating mechanism isprovided which checks for the existence of conditions indicative of amaximum load/low voltage condition and which automatically opens theexpansion valve to increase refrigerant flow. This has the beneficialeffect of cooling the system. By thus providing automatic system coolingit is possible to implement an HVAC system more economically since theheat pump motor can be sized and optimized for normal conditions insteadof abnormal conditions.

For a more complete understanding of the invention, its objects andadvantages, reference may be had to the following specification and tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram illustrating the basic componentsof an air-conditioning/heat pump system;

FIG. 2 is a more detailed view of the presently preferredair-conditioning/heat pump system into which the sensor fault handlingmechanism of the invention may be integrated;

FIGS. 3a and 3b comprise a flowchart depicting the presently preferredsensor check routine, FIGS. 3a and 3b being joined at the point depictedat "A";

FIGS. 4-7 are flowcharts depicting the sensor fault handling mechanismand method applicable when the heat pump system is in heating mode;

FIGS. 8-11 are flowchart diagrams depicting the sensor fault handlingmechanism and method applicable when the heat pump system is in coolingmode.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides an apparatus and method of handlingsensor faults. The presently preferred embodiment employs amicroprocessor-based control system with a complement of sensors and anelectronically controlled expansion valve to optimally control the flowof refrigerant through the system. To illustrate the principles of theinvention, a heat pump system, capable of providing both heating andcooling, will be described. A heat pump system of this type might besuitable for heating and cooling a commercial or residential building,although the principles of the invention are not limited to commercialand residential heating and cooling applications, but are applicable tovirtually all pumped heat transfer systems.

Before giving a detailed description of the presently preferredembodiment, a brief review of the refrigeration cycle may be helpful.That cycle will be described in connection with a basic heat pump systemillustrated schematically in FIG. 1.

The heat pump cycle uses the cooling effect of evaporation to lower thetemperature of the surroundings near one heat exchanger (the evaporator)and it uses the heating effect of high pressure, high temperature gas toraise the temperature of the surroundings near another heat exchanger(the condenser). This is accomplished by releasing a refrigerant underpressure (usually in the liquid phase ) into a low pressure region tocause the refrigerant to expand into a low temperature mixture of gasand liquid. Commonly, this low pressure region comprises an evaporatorcoil, such as evaporator coil 10. The refrigerant mixture, once in the,evaporator coil 10, is exposed to the high temperature ambient air ofthe region to be cooled. Evaporation of refrigerant from liquid to gasabsorbs heat from the ambient air and thereby cools it.

Release of refrigerant into the low pressure evaporator coil is usuallymetered by a restricted orifice or valve, commonly called an expansionvalve 12. There are a wide variety of different types of expansiondevices in use today, ranging from simple nonadjustable capillary tubes,to electrically adjustable valves such as pulse width modulated valves.The refrigerant at the outlet of the evaporator coil is compressed backinto a high pressure state by compressor 14 and condensed into a liquidphase by condenser 16 so that it may be used once again. If desired, asump 18 may be included, as illustrated. In a heat pump application,where the system is operating in the heating mode, the condensing ofhigh pressure gas into a liquid phase supplies heat to the surroundings.

Having reviewed the basic principles of the refrigeration or heat pumpcycle, a presently preferred embodiment of the invention will bedescribed. Although the invention can take many forms, it will bedescribed in conjunction with FIG. 2 where a heat pump system isdepicted generally at 20. The system includes an indoor unit 22, a roomunit or thermostat unit 23 and an outdoor unit 24. The indoor unitincludes an indoor coil or heat exchanger 26 and an indoor fan 28. Theindoor fan may be driven by a variable speed motor 30. The indoor fanand indoor coil are situated using suitable duct work, so that the fanforces ambient indoor air across the indoor coil at a rate determined bythe speed of the fan motor.

The outdoor unit includes an outdoor coil or heat exchanger 32 and anoutdoor fan 34 driven by suitable motor 36. Preferably, the outdoor unitincludes a protective housing which encases the outdoor coil and outdoorfan, so that the outdoor fan will draw ambient outdoor air across theoutdoor coil to improve heat transfer. The outdoor unit may alsotypically house a compressor 38.

The system illustrated in FIG. 2 is a so-called "heat pump" systembecause it can be used for both cooling and heating, by simply reversingthe function of the indoor coil and the outdoor coil. In the coolingmode, the outdoor coil functions as the condenser, while in the heatingmode, the outdoor coil functions as the evaporator. Switching betweenthe cooling mode and the heating mode is done using a four-way reversingvalve 40. Referring to FIG. 2, when the four-way valve is switched tothe cooling position (shown), the indoor coil functions as the condenserand the outdoor coil functions as the evaporator. When the four-wayvalve is set to the heating position (the alternate position), thefunctions of the coils are reversed.

The presently preferred embodiment uses an electronically controllableexpansion valve (EXV) 42. In the presently preferred embodiment theexpansion valve is a continuously variable (or incrementally variable)stepper motor valve which can be adjusted electronically to a wide rangeor orifice sizes or valve openings, ranging from fully open to fullyclosed. Although it is possible to implement the control system of theinvention with other types of electrically controlled valves, pulsewidth modulated valves being an example, the stepper motor valve ispresently preferred because it provides ripple-free operation andbecause it is more trouble-free. The stepper motor valve only needs tomove or "cycle" when an orifice size adjustment is made. This may happenseveral times during a typical operating sequence (e.g., several timesper hour). In contrast, the pulse width modulated valve cyclescontinuously at high frequency during the entire operating sequence.

The presently preferred system is a microprocessor-based system whichgathers data from various sensors and which, among other things,determines the proper setting of the expansion valve based on the datagathered. This same microprocessor-based system is also used to performthe airflow obstruction and blocked fan detection processes of theinvention, as will be more fully described below. More specifically, thepresently preferred embodiment uses three interconnectedmicroprocessor-based control units 44, 45 and 46, associated with theoutdoor unit 24, indoor unit 22 and room unit or thermostat unit 23,respectively. Preferably all three microprocessor-based control unitsare linked together via a suitable communication link 48, such as aparallel or serial communication link. The outdoor control unit 44, is,in part, responsible for data collection, while the indoor control unit46 is responsible for: on/off cycling of the system, modulating theindoor fan speed, control of the expansion valve, start/termination ofdemand defrost cycle, system diagnostics and performing the blocked fanand airflow obstruction detection processes of the invention.

The microprocessor-based system employs a plurality of sensors formeasuring temperature at various locations throughout the system.Specifically, the present invention has a first temperature sensor 54,which measures the discharge temperature of the refrigerant as it exitsthe compressor 38. A second temperature sensor 55 measures thetemperature of the outdoor heat exchanger 32 and a third temperaturesensor 56 measures the temperature of the ambient air that is drawn intoheat exchange contact with the outdoor heat exchanger by action of fan34. Preferably temperature sensor 56 is situated in an inset region ofthe outdoor unit housing, so that it will be shaded from direct sunlightand yet will be situated in the airflow path of the air which makes heatexchange contact with the outdoor heat exchanger 32. In addition tothese sensors, the system may also employ a fourth temperature sensor 60which may be integrated into the room unit or thermostat unit 23. Ifdesired, a humidity sensor 62 may also be incorporated in the room unit23 and a voltage sensor 162 and current sensor 163 can be connected atany suitable points to sense the AC line voltage and current.

As the system operates it is expected that a certain amount of frostwill begin to build up on the heat exchanger which is functioning as theevaporator. In a heat pump application, in heating mode, the outdoorcoil functions as the evaporator. Thus, in the heating mode theevaporator coil will gradually build up an accumulation of frost. Thisbuildup of frost degrades system performance by obstructing optimal heattransfer. The present embodiment employs a demand defrost system whichis designed to periodically melt this accumulated frost. Essentially,when a frost buildup is detected (as discussed below), four-wayreversing valve 40 is cycled to its opposite position, temporarilyreversing the functions of the indoor and outdoor coils. This causesheat to be pumped to the frosted outdoor coil, melting the frost.

The presently preferred embodiment uses thermistor sensors astemperature sensors. Thermistors are preferred because they arehermetically sealed, solid-state devices, with a low probability ofmechanical-type failure. In addition, thermistors provide long termstability and good temperature measurement accuracy over a widetemperature range. Although thermistors are presently preferred, othertypes of sensors may also be used to implement the invention.

The presently preferred sensor fault detection routine or sensor checkroutine is depicted in FIG. 3. The sensor check routine begins at step120, by comparing the outdoor air temperature with a predetermined lowtemperature, preferably -77° F. If the outdoor air temperature is notgreater than this value, a software flag is set indicating the outdoorair thermistor i s bad. This is illustrated at step 122. Next, at step124 the outdoor coil temperature is compared with a predetermined value,again -77° F. If the outdoor coil temperature is not greater than thispredetermined low value, then a flag is set indicating the outdoor coilthermistor as bad. This is illustrated at step 126. Next, at step 128,the temperature reading from the compressor discharge temperature sensoris compared with a predetermined value, in this case O° F. If thecompressor discharge temperature reading is not greater than thispredetermined value, then a software flag is set indicating thecompressor discharge thermistor as bad. This is shown at step 130. Thesesoftware flags are used by the sensor fault handling mechanism andmethod of the invention. While the sensor check routine of FIG. 3 ispresently preferred for its simplicity and reliability, other sensorintegrity checks may alternatively be employed. For example, if desired,the impedance of each sensor can be tested in order to identify anyshort circuited or open circuited sensors.

After the sensor check routine has set any applicable flags, controlproceeds to step 220 where a series of tests are performed to determinedwhether a maximum load condition exists. First, at step 220 the systemtests to see if the outdoor air temperature reading is above 100° F. Ifnot, the system is operated normally as indicated at 222. If the outdoorair temperature reading is greater than 100° F. (or some othercomparatively high temperature indicative of a maximum load condition),control proceeds to step 224 where an alert flag is set. After the alertflag is set, one or more of a series of additional tests may beperformed. In FIG. 3b the system tests the compressor dischargetemperature at step 226 to determine if the discharge temperature isgreater than 250° F. If the result of this test is TRUE, then controlproceeds to step 232 where the alert flag is tested. If the alert flagis set (i. e., from step 224), then the setting of the expansion valveEXV is opened or increased to allow an increased flow of refrigerantthrough the compressor. If the alert flag was not set as determined atstep 232, control simply proceeds to step 222 where the system operatesnormally.

The flowchart of FIG. 3b illustrates two additional tests at steps 228and 230 which may be optionally included, separately or together, asadditional tests or as a substitution for the test performed at step226. In step 228 a test is performed to see if the incoming line voltageis low. A low line voltage condition would be indicative, for example,of a brownout condition. If the line voltage is low, control proceeds tostep 232 where the alert flag is tested as described above. Step 230tests to determine whether the compressor current is high. If thecurrent is high this can be indicative of a maximum load or brownoutcondition and control similarly proceeds to step 232 where the alertflag is tested. It should be understood that the flowchart of FIG. 3billustrates several redundant tests useful in determining when themaximum load condition exists. In an actual commercial embodiment it maynot be necessary to include all of these tests. For example, suitableresults could be obtained using only tests 220 and 226, or tests 220 and228, or tests 220 and 230, for example. Also, while the use of an alertflag has been illustrated, this is but one way of implementing the logicperformed by the system of the invention. Other equivalent flowchartscan be constructed to accomplish the same results.

By increasing the refrigerant flow in response to sensing the existenceof or onset of a maximum load or low voltage condition the additionalrefrigerant flow causes the system to automatically begin cooling themotor. In this way, a smaller motor can be used without worry ofoverheating, and the smaller motor can therefore be properly optimizedto provide maximum efficiency during normal operating conditions(non-maximum load). Thus, in effect, the electronics of the presentinvention automatically cool the motor during the maximum load and lowvoltage conditions, so that lower cost or efficient motors can be used.

The presently preferred sensor fault handling technique is summarized inTable I and Table II. Table I depicts the expansion valve controlmethod, the defrost control method and the indoor fan speed controlmethod for a heat pump system operating in heating mode. Themicroprocessor-based control system described above reads the state ofthe thermistor flags as set by the sensor check routine of FIG. 3 andselects the appropriate operating mode based on whether the sensors havebeen flagged as faulty or not. More specifically, themicroprocessor-based system selects the operating mode of the expansionvalve control, the operating mode of the defrost mechanism and theoperating mode of the indoor fan speed setting based on these sensorflags. Table II illustrates the manner in which the microprocessor-basedsystem selects the operating mode of the expansion valve control and ofthe indoor fan speed setting when the system is in the cooling mode. Forconvenience, Tables I and II both include a column captioned "Reference"to direct the reader to the figure which illustrates that particularoperating mode.

Referring to FIGS. 4-7, when the heat pump system is in a heating mode(as illustrated in each of FIGS. 4-7) the illustrated routines areperformed. It is noted that when all sensors are operating properly, theexpansion valve control is operated automatically in a closed-loopfeedback system based on the compressor discharge temperature usingoutdoor air temperature as a reference. While the use of compressordischarge temperature and outdoor air temperature is presently preferredin the automatic control of the expansion valve, other sensor locationsand arrangements are also possible. Further, with all sensors operatingproperly, the defrost method employs a demand defrost scheme whereby theoutdoor coil is defrosted on an as-needed basis, based on outdoor coiltemperature. Finally, with all sensors operating properly the indoor fanspeed is preferably modulated based on outdoor air temperature. Ideally,the fan speed is set to the appropriate level in order to optimizeefficiency.

FIG. 4 illustrates the condition in which all sensors are properlyoperating in heating mode. This corresponds to the branch beginning atstep 132 entitled "Normal System Operation." The case in which all threesensors have been flagged as bad is also depicted in FIG. 4 beginningwith step 134 at which the malfunction code is displayed on the roomthermostat. With all three sensors flagged as faulty, the expansionvalve is set to a fixed position (step 136), the indoor fan speed is setto its maximum speed (step 138) and the demand defrost routine isdisabled (step 142). In addition, the compressor discharge control isdisabled so that the unit will continue to operate with the expansionvalve set to the fixed default setting. This is illustrated at step 140.

The remaining FIGS. 5-11 show other combinations of faulty sensors andproperly operating sensors. FIGS. 4-7 pertain to the Heating Mode;whereas FIGS. 8-11 pertain to the Cooling Mode.

By way of comparison, FIG. 8 illustrates the case in which all threesensors are operating properly in the cooling mode. FIG. 8 also depictsthe case in which all three sensors have been flagged as failed in thecooling mode. The remaining figures show the microprocessor-selectedmodes of operating the expansion valve, the demand defrost and theindoor fan speed for each of the remaining sensor OK/NOT OKpermutations. It is noted that in the cooling mode, the presentlypreferred embodiment does not implement demand defrost control. Ofcourse, this feature could be added, if desired, by simply extending thelogic illustrated for the heating mode in Table I.

                                      TABLE I                                     __________________________________________________________________________           FIG. 4  FIG. 5  FIG. 6  FIG. 7                                         __________________________________________________________________________    Operation                                                                            Heat    Heat    Heat    Heat                                           Mode                                                                          Discharge                                                                            Not OK.sup.2                                                                          Not OK.sup.2                                                                          OK.sup.1                                                                              Not OK.sup.2                                   Sensor OK.sup.1                                                                              OK.sup.1                                                                              Not OK.sup.2                                                                          OK.sup.1                                       Outdoor Coil                                                                         Not OK.sup.2                                                                          OK.sup.2                                                                              OK.sup.1                                                                              OK.sup.2                                       Sensor OK.sup.1                                                                              Not OK.sup.1                                                                          Not OK.sup.2                                                                          Not OK.sup.1                                   Outdoor Air                                                                          Not OK.sup.2                                                                          Not OK.sup.2                                                                          Not OK.sup.1                                                                          OK.sup.2                                       Sensor OK.sup.1                                                                              OK.sup.1                                                                              OK.sup.2                                                                              Not OK.sup.1                                   EXV Control                                                                          Default - Fixed                                                                       Default - Fixed                                                                       Auto -  Fixed EXV                                      Method EXV Opening.sup.2                                                                     EXV Opening.sup.2                                                                     Discharge                                                                             Opening -                                             Auto -  Auto -  Temp. Control.sup.1                                                                   Function of                                           Discharge                                                                             Discharge                                                                             Fixed EXV                                                                             Outdoor Air                                           Temp. Control.sup.1                                                                   Temp. Control.sup.1                                                                   Opening -                                                                             Temp..sup.2                                                           Function of                                                                           Auto -                                                                Outdoor Air                                                                           Discharge                                                             Temp..sup.2                                                                           Temp.                                                                         Control.sup.1                                  Defrost -                                                                            90 Min. - Frost                                                                       90 Min. - Frost                                                                       90 Min. - Frost                                                                       Demand                                         Method Timed   Temp.   Temp.   Defrost.sup.2                                         Termination.sup.2                                                                     Termination.sup.2                                                                     Termination.sup.1                                                                     90 Min. -                                             Demand.sup.1                                                                          90 Min. -                                                                             90 Min. -                                                                             Frost-Timed                                           Defrost Frost-Timed                                                                           Frost-Timed                                                                           Termination.sup.1                                             Termination**.sup.1                                                                   Termination*.sup.2                                     Indoor Fan                                                                           Maximum.sup.2                                                                         Maximum.sup.2                                                                         Maximum.sup.1                                                                         Based on                                       Speed Setting                                                                        Based on                                                                              Based on                                                                              Based on                                                                              Outdoor                                               Outdoor Outdoor Outdoor Temp..sup.1                                           Temp..sup.1                                                                           Temp..sup.1                                                                           Temp..sup.2                                                                           Maximum.sup.2                                  __________________________________________________________________________     .sup.1&2 See FIGS. 4 through 7 for Explanation.                               **Defrosting Period is a Function of Outdoor Air Temperature.            

                  TABLE II                                                        ______________________________________                                        FIG. 8        FIG. 9    FIG. 10    FIG. 11                                    ______________________________________                                        Operation                                                                             Cool      Cool      Cool     Cool                                     Discharge                                                                             Not OK.sup.4                                                                            Not OK.sup.4                                                                            OK.sup.3 Not OK.sup.4                             Mode                                                                          Sensor  OK.sup.3  Ok.sup.3  Not OK.sup.4                                                                           OK.sup.3                                 Outdoor Not OK.sup.4                                                                            OK.sup.4  OK.sup.3 OK.sup.4                                 Coil    OK.sup.3  Not OK.sup.3                                                                            Not OK.sup.4                                                                           Not OK.sup.3                             Sensor                                                                        Outdoor Not OK.sup.4                                                                            Not OK.sup.4                                                                            Not OK.sup.3                                                                           OK.sup.4                                 Air Sensor                                                                            OK.sup.3  OK.sup.3  OK.sup.4 Not OK.sup.3                             EXV     Default - Default - Default -                                                                              Fixed EXV                                Control Fixed EXV Fixed EXV Fixed EXV                                                                              Opening -                                Method  Opening.sup.4                                                                           Opening.sup.4                                                                           Opening.sup.3                                                                          Function of                                      Auto -    Auto -    Fixed EXV                                                                              Outdoor Air                                      Discharge Discharge Opening -                                                                              Temp..sup.4                                      Temp.     Temp.     Function of                                                                            Default -                                        Control.sup.3                                                                           Control.sup.3                                                                           Outdoor Air                                                                            Fixed EXV                                                            Temp..sup.4                                                                            Opening.sup.3                            Indoor Fan                                                                            Maximum.sup.4                                                                           Maximum.sup.4                                                                           Maximum.sup.3                                                                          Based on                                 Speed   Based on  Based on  Based on Outdoor                                  Setting Outdoor   Outdoor   Outdoor  Temp..sup.4                                      Temp..sup.3                                                                             Temp..sup.3                                                                             Temp..sup.4                                                                            Maximum.sup.3                            ______________________________________                                         .sup.3&4 See FIGS. 8 through 11 for Explanation.                         

The sensor fault handling mechanism and method illustrated above isdesigned to automatically select the operating mode of key systemcomponents so that sensor malfunction will have a minimum negativeimpact on system performance and reliability. While the invention hasbeen illustrated in its presently preferred form, it will be understoodthat the invention is capable of modification or change withoutdeparting from the spirit of the invention as set forth in the appendedclaims.

We claim:
 1. In a heat pump system powered by an electrical line,including a refrigerant compressor and an electrically controlledexpansion valve, a system operating method comprising,obtaining anoutdoor air temperature by measuring a property indicative of theoutdoor air temperature; obtaining a line voltage by measuring aproperty indicative of line voltage; comparing the outdoor airtemperature with a predetermined temperature indicative of an alertcondition; comparing the line voltage to a predetermined valueindicative of a low voltage condition, and based on said comparingsteps: if the outdoor air temperature is above said predeterminedtemperature and if the line voltage is below said predetermined value,then automatically increasing the setting of said expansion valve toincrease the flow of refrigerant in said heat pump system.
 2. The systemof claim 1 wherein said heat pump system includes an outdoor unit andwherein said outdoor air temperature is obtained by measuring theambient air temperature at said outdoor unit.
 3. The system of claim 1wherein said outdoor air temperature is obtained by measuring thedischarge temperature of refrigerant discharging from said compressorand using said discharge temperature as an indication of outdoor airtemperature.
 4. In a heat pump system including a refrigerant compressorand an electrically controlled expansion valve, a system operatingmethod comprising:obtaining an outdoor air temperature by measuring aproperty indicative of the outdoor air temperature; obtaining adischarge temperature by measuring a property indicative of thetemperature of refrigerant discharged from the compressor; comparing theoutdoor air temperature with a first predetermined temperatureindicative of an alert condition; comparing the discharge temperature toa second predetermined temperature indicative of an alert condition; andbased on said comparing steps: if the outdoor air temperature is abovesaid first predetermined temperature and if the discharge temperature isabove said second predetermined temperature then automaticallyincreasing the setting of said expansion valve to increase the flow ofrefrigerant in said heat pump system.
 5. The system of claim 4 whereinsaid heat pump system includes an outdoor unit and wherein said outdoorair temperature is obtained by measuring the ambient air temperature atsaid outdoor unit.
 6. In a heat pump system including a refrigerantcompressor powered by electric current and an electrically controlledexpansion valve, a system operating method comprising,obtaining anoutdoor air temperature by measuring a property indicative of theoutdoor air temperature; obtaining a current signal by measuring aproperty indicative of the current powering said compressor; comparingthe outdoor air temperature with a predetermined temperature indicativeof an alert condition; comparing the current signal to a predeterminedvalue indicative of an alert condition, and based on said comparingsteps: if the outdoor air temperature is above said predeterminedtemperature and if the current signal is above said predetermined value,then automatically increasing the setting of said expansion valve toincrease the flow of refrigerant in said heat pump system.
 7. The systemof claim 6 wherein said heat pump system includes an outdoor unit andwherein said outdoor air temperature is obtained by measuring theambient air temperature at said outdoor unit.
 8. The system of claim 6wherein said outdoor air temperature is obtained by measuring thedischarge temperature of refrigerant discharging from said compressorand using said discharge temperature as an indication of outdoor airtemperature.